When to use -O2 flag for gcc?
I would recommend using -O2 most of the time, benefits include:
- Usually reduces size of generated code (unlike -O3).
- More warnings (some warnings require analysis that is only done during optimization)
- Often measurably improved performance (which may not matter).
If release-level code will have optimization enabled, it's best to have optimization enabled throughout the development/test cycle.
Source-level debugging is more difficult with optimizations enabled, occasionally it is helpful to disable optimization when debugging a problem.
I'm in bioinformatics so my advice may be biased. That said, I always use the -O3 switch (for release and test builds, that is; not usually for debugging). True, it has certain disadvantages, namely increasing compile-time and often the size of the executable.
However, the first factor can be partially mitigated by a good build strategy and other tricks reducing the overall build time. Also, since most of the compilation is really I/O bound, the increase of compile time is often not that pronounced.
The second disadvantage, the executable's size, often simply doesn't matter at all.